Investment Securities (Contractual maturities) (Details) $ in Millions	Mar. 31, 2017 USD ($)
Amortized cost
Within one year	$ 7,446
After one year through five years	5,029
After five years through ten years	5,134
After ten years	17,721
Estimated fair value
Within one year	7,446
After one year through five years	5,215
After five years through ten years	5,556
After ten years	$ 20,673